HEMT having conduction barrier between drain fingertip and source

ABSTRACT

A High Electron Mobility Transistor (HEMT) includes an active layer on a substrate, and a Group IIIA-N barrier layer on the active layer. An isolation region is through the barrier layer to provide at least one isolated active area including the barrier layer on the active layer. A gate is over the barrier layer. A drain includes at least one drain finger including a fingertip having a drain contact extending into the barrier layer to contact to the active layer and a source having a source contact extending into the barrier layer to contact to the active layer. The source forms a loop that encircles the drain. The isolation region includes a portion positioned between the source and drain contact so that there is a conduction barrier in a length direction between the drain contact of the fingertip and the source.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/864,157, filed Jan. 8, 2018, issued as U.S. Ser. No. 10/680,093B1,which is a divisional of U.S. patent application Ser. No. 15/353,857,filed Nov. 17, 2016, issued as U.S. Pat. No. 9,882,041B1, both of whichare incorporated herein by reference in their entireties.

FIELD

Disclosed embodiments relate to Group IIIA-N (e.g., Gallium Nitride)High Electron Mobility Field Effect Transistors (HEMTs).

BACKGROUND

Gallium-nitride (GaN) is a commonly used Group IIIA-N material forelectronic devices, where Group IIIA elements such as Ga (as well asboron, aluminum, indium, and thallium) are also sometimes referred to asGroup 13 elements. GaN is a binary IIIA/V direct band gap semiconductorthat has a Wurtzite crystal structure. Its relatively wide band gap of3.4 eV at room temperature (vs. 1.1 eV for silicon at room temperature)affords it special properties for a wide variety of applications inoptoelectronics, as well as high-power and high-frequency electronicdevices.

GaN-based HEMTs are known which feature a junction between two materialswith different band gaps to form a heterojunction or heterostructure.The HEMT structure is based on a very high electron mobility, describedas a two-dimensional electron gas (2DEG) which forms just below aheterostructure interface between a barrier layer (that typicallycomprises AlGaN) on a generally intrinsic active layer (that typicallycomprises GaN) due to the piezoelectric effect and a naturalpolarization effect. As with any power FET device, there are a gate,source electrode, and drain electrode, where the source electrode anddrain electrode each include contacts that extend through the topbarrier layer to form an ohmic contact with the underlying 2DEG in thesurface of the active layer. One Group IIIA-N HEMT layout is a draincentered layout where the high voltage drain area is completely enclosedby the gate and by the source. This layout has advantages includingregarding device isolation, edge termination, and leakage currentcontrol.

SUMMARY

This Summary is provided to introduce a brief selection of disclosedconcepts in a simplified form that are further described below in theDetailed Description including the drawings provided. This Summary isnot intended to limit the claimed subject matter's scope.

Disclosed embodiments recognize for drain centered Group IIIA-N HEMTduring device operation near the drain contact of the fingertip of thedrain finger there is generally a high concentration of ‘hot’ carriersthat can degrade and destroy (e.g., melt) the device especially underhigh power switching conditions. By providing an isolation region thatis through the barrier layer around the drain contact fingertip of thedrain finger so that there is a resulting conduction barrier provided ina length direction of the drain finger between the drain contact of thefingertip and the source, the conduction barrier suppresses the hotcarrier injection problem, thus improving device robustness andreliability.

The isolation region can be formed by patterning the barrier layer toform an active area Mesa structure having adjacent conduction barriers,or can comprise an implanted isolation region with no need for etchingthe barrier layer. In the case of an active area Mesa having an adjacentconduction barrier, the Mesa boundary can be rounded by using agreyscale mask to suppress leakage or breakdown that may otherwise occuralong the Mesa edge.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made to the accompanying drawings, which are notnecessarily drawn to scale, wherein:

FIGS. 1A-C provide successive top perspective cross sectional views thatcorrespond to steps in an example method for forming a Group IIIA-N HEMThaving an isolation region including a portion positioned between thesource and the drain contact so that there is a conduction barrier in alength direction of the drain finger between the drain contact fingertipand the source, according to an example embodiment. The view in FIG. 1Cis rotated 180 degrees from the views shown in FIGS. 1A and 1B, and isonly showing a portion of the active areas shown in FIGS. 1A and 1B.

FIG. 2 is a top view of a Group IIIA-N HEMT having a drain fingertip onan isolation region, according to an example embodiment.

FIG. 3A is a zoomed in top view of a portion of a known Group IIIA-NHEMT having a drain centered layout.

FIG. 3B is a zoomed in top view of a portion of a first example GroupIIIA-N HEMT having a drain centered layout with an isolation regionincluding a portion positioned between the source and the drain contactso that there is a conduction barrier in a length direction of the drainfinger between the drain contact of the fingertip and the source.

FIG. 3C is a zoomed in top view of a portion of a second example GroupIIIA-N HEMT having a drain centered layout with an isolation regionincluding a portion positioned between the source and the drain contactso that there is a conduction barrier in a length direction of the drainfinger between the drain contact of the fingertip and the source.

FIG. 3D is a zoomed in top view of a portion of another Group IIIA-NHEMT having a drain centered layout with an isolation region includingan isolation region portion positioned between the source and the draincontact so that there is a conduction barrier in a length direction ofthe drain finger between the drain contact of the fingertip and thesource. The HEMT features its drain contact extending in the lengthdirection of the drain finger beyond the source contact to furthersuppress the current crowding effect.

FIG. 4 shows actual 600V hard switching yield data that compares resultsfrom a Group IIIA-N HEMT having a known drain centered layout and adisclosed Group IIIA-N HEMT having a drain centered layout with a draincontact on an isolation region including a portion positioned betweenthe source and the drain contact so that there is a conduction barrierin a length direction of the drain finger between the drain contactfingertip and the source.

DETAILED DESCRIPTION

Example embodiments are described with reference to the drawings,wherein like reference numerals are used to designate similar orequivalent elements. Illustrated ordering of acts or events should notbe considered as limiting, as some acts or events may occur in differentorder and/or concurrently with other acts or events. Furthermore, someillustrated acts or events may not be required to implement amethodology in accordance with this disclosure.

Also, the terms “coupled to” or “couples with” (and the like) as usedherein without further qualification are intended to describe either anindirect or direct electrical connection. Thus, if a first device“couples” to a second device, that connection can be through a directelectrical connection where there are only parasitics in the pathway, orthrough an indirect electrical connection via intervening itemsincluding other devices and connections. For indirect coupling, theintervening item generally does not modify the information of a signalbut may adjust its current level, voltage level, and/or power level.

FIGS. 1A-C provide successive top perspective cross sectional views thatcorrespond to steps in an example method for forming a disclosed GroupIIIA-N HEMT having an isolation region including a portion positionedbetween the source and the drain contact so that there is a conductionbarrier in a length direction of the drain contact of the drain contactfingertip and the source, according to an example embodiment. As notedabove, the view in FIG. 1C is rotated 180 degrees from the view shown inFIGS. 1A and 1B, and is only showing a portion of the active areas shownin FIGS. 1A and 1B.

An in-process HEMT is shown in FIG. 1A comprising a substrate 102, atleast one Group IIIA-N buffer layer 103 on the substrate 102, a GroupIIIA-N active layer 104 on the buffer layer 103, and a Group IIIA-Nbarrier layer 106 on the active layer 104. As known in the art, a 2DEGis formed in the active layer 104 near its heterojunction throughout itsinterface with the barrier layer 106. The barrier layer 106, the activelayer 104 and the buffer layer 103 are generally all epitaxial layers onthe substrate 102. A patterned masking material 108 (e.g., photoresist)is shown on the barrier layer 106 that is used to define the isolationregions 110 which defines the active areas. The T-shape pattern shapeshown is only for example.

The substrate 102 can comprises sapphire, silicon, silicon carbide (SiC)or GaN. The Group IIIA-N buffer layer 103 is generally present on thesubstrate 102, but is not needed when a gallium nitride (GaN) substrateis used. The active layer 104 can comprise, for example, 25 to 1,000nanometers of GaN. The active layer 104 may be formed so as to minimizecrystal defects which may have an adverse effect on electron mobility.The active layer 104 is commonly undoped (e.g., undoped GaN).

The barrier layer 106 can comprise, for example, 8 to 30 nanometers ofAl_(x)Ga_(1-x)N or In_(x)Al_(y)Ga_(1-x-y)N. A composition of Group IIIAelements in the barrier layer 106 may be, for example, 24 to 28 percentatomic weight aluminum nitride and 72 to 76 percent atomic weightgallium nitride. Forming the barrier layer 106 on the active layer 104generates a 2DEG in the active layer 104 throughout its interface withthe barrier layer 106 just below the barrier layer 106, with an electrondensity of, for example, 1×10¹² to 2×10¹³ cm⁻². The barrier layer 106may include an optional capping layer, for example comprising GaN, on atop surface of the barrier layer 106.

The patterned masking material 108 functioning as an isolation mask isused to form isolation regions 110 that define at least isolated activearea from the barrier layer 106 and active layer 104, with the resultsshown in FIG. 1B showing two active area 106/104. FIG. 1B showsisolation regions 110 that lack the barrier layer 106 which surround anisolated active area 106/104 having the barrier layer 106 on the activelayer 104 to provide the 2DEG. The isolation mask using the patternedmasking material 108 may include, for example, 200 nanometers to 2microns of photoresist formed by a photolithographic process. Theforming the isolation regions 110 can comprises a Mesa etch process. Forexample, a blanket barrier layer 106 can be patterned using a greyscalemask followed by an etch to provide rounded edges. As shown in FIG. 1B,this Mesa etch process besides etching through the barrier layer 106also removes a portion of the active layer 104.

As known in the art, using a greyscale mask enables micro-lithographersto shape resist using a single exposure with a custom attenuating maskcomprised of sub-resolution pixels. The isolation process usingpatterned masking material 108 may also be an isolation implant(s) whichselectively implants dopants into the barrier layer 106 and into theactive layer 104 to form a heavily doped isolation barrier. In eithercase the isolation region 110 functions as a conduction barrier thatreduces or eliminates electrical current in the 2DEG from crossingtherethrough.

A gate 114, drain contact 120 b and a source 122 with source contact 122a are formed within the active areas 106/104 with the results shown inFIG. 1C for a portion of active areas 106/104 that include a portion ofa drain contact 120 b with its drain finger 120 a including the drainfingertip 120 a 1 and a portion of the source 122 and source contact 122a shown. As noted above, the view in FIG. 1C is rotated 180 degrees fromthe view shown in FIGS. 1A and 1B, and each active area 106/104 is onlya portion of what is shown in FIGS. 1A and 1B. The gate 114 is shownformed over the barrier layer 106. The source contacts and draincontacts are generally formed by a masked etch process that selectivelyetches part of the thickness of the barrier layer 106 to extend into thebarrier layer 106 to provide good (low resistance) contact to the 2DEGin the active layer 104 near the interface between the barrier layer 106and the active layer 104.

Although not shown in the view provided, the source 122 forms a completeloop that encircles the drain (see FIG. 2 for a source 122 providingencirclement of the drain 120). In FIG. 1C an example active areaboundary 141 is shown as well as the position of a known (prior art)active area boundary 147. The position of the active area boundary 141results in the fingertip 120 a ₁ being over isolation region 110including an isolation region portion positioned between the source 122and the drain contact 120 b so that there is a conduction barrier in alength direction of the drain contact 120 b in the drain finger 120 abetween the drain contact of the fingertip 120 a ₁ and the source 122.In contrast, the position of the known active area boundary 147 resultsin the fingertip 120 a ₁ being over the active area 106/104 so thatthere is no conduction barrier in a length direction of the draincontact 120 b in the drain finger 120 a between the drain contact of thefingertip 120 a ₁ and the source 122, which undesirably results in hotcarriers flowing between the source 122 and the drain contact 120 b inthe fingertip 120 a during device operation as described above.

The gate 114, drain including drain contact 120 b and the sourceincluding the source contact 122 a all generally comprise a metal, suchas a TiW alloy in one particular embodiment. The respective electrodescan be formed by sputtering a metal stack, such as Ti/Al/TiN in anotherparticular embodiment. Although not shown in FIG. 1C, the source and thedrain metal layers are generally on top of a dielectric layer that is ontop of the barrier layer 106 and over the gate to prevent shorts to thegate.

FIG. 2 is a top view of an integrated circuit (IC) 250 including a2-finger Group IIIA-N HEMT 200 having an isolation region 110 includingan isolation portion positioned between the source 122 and the draincontact 120 b of the drain 120 so that there is a conduction barrier ina length direction of the drain contact 120 b in the drain fingers 120 abetween their fingertip 120 a 1 and the source 122, according to anexample embodiment. IC 250 also includes other circuitry shown as blocks240 and 245 on another active area 106/104, such as for realizing aDC/DC power converter IC. Although not explicitly shown in FIG. 2 (aswell as FIGS. 3A-3D), there is electrical isolation between the metalproviding the gate 114 and the source 122. In one embodiment there is adielectric layer over the metal of the gate 114, where the metal of thesource 122 is on top of the dielectric layer on the gate metal to formfield plates. In another embodiment, there is a spacing between themetal of the source 122 and the gate 114 so that this dielectric layerbetween the source and gate metal is not needed.

The area of the drain 120 is shown completely enclosed by both the gate114 and by the source 122. The active area 106/104 can be seen to beenclosed by the isolation region 110 which includes only active layer104 (no barrier layer 106). The active area 106/104 in this embodimentcan comprise a Mesa (raised in height due to unremoved barrier layer 106on the active layer 104) as compared to the isolation region 110 whichin the Mesa embodiment lacks at least the barrier layer 106. The gate is114. The example active area boundary 141 (also in FIG. 2) is shownbelow the drain contact 120 b.

FIG. 3A is a zoomed in top view of a portion of a known Group IIIA-NHEMT 300 (marked prior art) having a drain centered layout. Electronflow during operation of HEMT 300 is shown by the arrows extending in a180 degree arc in a path from the source 122 to the drain contact 120 bin the fingertip 120 a 1 of the drain finger 120 a. This electron flowpattern is recognized to result in a high concentration of ‘hot’carriers that can degrade and destroy (e.g., melt) the HEMT deviceespecially under high current switching conditions.

FIG. 3B is a zoomed in top view of a portion of a Group IIIA-N HEMT 320having a drain centered layout with an isolation region 110 including anisolation region portion 110′ positioned between the source 122 and thedrain contact 120 b so that there is a conduction barrier in a lengthdirection of the drain finger 120 a between drain contact 120 b of thefingertip 120 a 1 and the source 122. By providing an isolation regionportion 110′ that is through the barrier layer 106 to expose at leastthe active layer 104 around the fingertip 120 a 1 of the drain finger120 a so that there is a resulting conduction barrier provided in alength direction of the drain finger between the drain contact 120 b ofthe fingertip 120 a 1 and the source 122, the conduction barriersuppresses the hot carrier injection problem, thus improving devicerobustness and reliability. The conduction barrier 110′ is shownblocking conduction through at least a 150 degree arc in a path from thesource 122 to the drain contact 120 b of the fingertip 120 a 1.

FIG. 3C is a zoomed in top view of a portion of a Group IIIA-N HEMT 340having a drain centered layout with an isolation region 110 including anisolation region portion 110″ positioned between the source 122 and thedrain contact 120 b so that there is a conduction barrier in a lengthdirection of the drain finger 120 a between the drain contact 120 b ofthe fingertip 120 a 1 and the source 122. The pulled back position ofthe active area boundary 141 results in the isolation cut line and thusthe isolation region portion 110″ being separated from, shown beingbelow, the drain contact 120 b. By providing an isolation region portion110′ that is through the barrier layer 106 to expose the active layer104 around the fingertip 120 a 1 of the drain finger 120 a so that thereis a resulting conduction barrier 110″ provided in a length direction ofthe drain finger between the drain contact 120 b of the fingertip 120 a1 and the source 122, the hot carrier injection problem is suppressed,thus improving device robustness and reliability.

FIG. 3D is a zoomed in top view of a portion of another Group IIIA-NHEMT 370 having a drain centered layout with an isolation region 110including an isolation region portion 110″ positioned between the source122 and the drain contact 120 b so that there is a conduction barrier ina length direction of the drain finger 120 a between the drain contact120 b in the fingertip 120 a 1 and the source 122. As in FIG. 3C, thepulled back position of the active area boundary 141 results in theisolation cut line and thus the isolation region portion 110″ beingseparated from, shown being below, the drain contact 120 b. HEMT 370features its drain contact 120 b extending in the length direction ofthe drain finger 120 a beyond the source contact 122 a to furthersuppress the current crowding effect, and is otherwise analogous to theHEMT 340 shown in FIG. 3C. The drain contact extension distance shown as375 can be from about 1 μm to about 100 μm.

Disclosed HEMTs apply to both enhancement and depletion mode devices.Disclosed HEMTs can be embodied as discrete devices or on ICs such aspower converters (e.g., DC/DC converters) and power switches.

EXAMPLES

Disclosed embodiments are further illustrated by the following specificExamples, which should not be construed as limiting the scope or contentof this Disclosure in any way.

FIG. 4 shows actual 600V hard switching yield data that compares resultsfrom a known Group IIIA-N HEMT having a drain centered layout (shown as“Prior Art”) and a disclosed Group IIIA-N HEMT having a drain centeredlayout with a disclosed isolation region 110 including a portionpositioned between the source and drain contact (shown as “New”) so thatthere is a conduction barrier in a length direction of the drain fingerbetween the drain contact 120 b of the fingertip 120 a 1 and the source122. The switching yield can be seen to be >99% for the disclosed GroupIIIA-N HEMT as compared to about 72% for the known disclosed GroupIIIA-N HEMT.

Disclosed embodiments can be used to form semiconductor die that may bediscrete devices or part of integrated circuits integrated into avariety of assembly flows to form a variety of different devices andrelated products. The semiconductor die may include various elementstherein and/or layers thereon, including barrier layers, dielectriclayers, device structures, active elements and passive elementsincluding source regions, drain regions, bit lines, bases, emitters,collectors, conductive lines, conductive vias, etc. Moreover, thesemiconductor die can be formed from a variety of processes includingbipolar, Insulated Gate Bipolar Transistor (IGBT), CMOS, BiCMOS andMEMS.

Those skilled in the art to which this disclosure relates willappreciate that many other embodiments and variations of embodiments arepossible within the scope of the claimed invention, and furtheradditions, deletions, substitutions and modifications may be made to thedescribed embodiments without departing from the scope of thisdisclosure.

The invention claimed is:
 1. An electronic device, comprising: an active area formed by a group IIIA-N barrier layer over a group IIIA-N active layer; a source electrode formed over the active area and having a first portion and a second portion, the first and second portions being parallel and spaced apart, and a third portion connecting the first and second portions; a drain electrode formed over the active area and having a drain finger located between the first and second source electrode portions, the drain finger having first and second parallel sides connected via a drain fingertip sidewall; and an active area boundary between the active area and an isolation region, the active area boundary located between the drain fingertip sidewall and the third portion of the source electrode.
 2. The electronic device as recited in claim 1, wherein the active area boundary is defined by an edge of a mesa structure.
 3. The electronic device as recited in claim 2, wherein the edge of the mesa structure is rounded.
 4. The electronic device as recited in claim 1, further comprising a plurality of drain contacts connected to the drain finger, wherein the active area boundary is located between the drain fingertip sidewall and the drain contact nearest to the drain fingertip sidewall.
 5. The electronic device as recited in claim 1, wherein the active area boundary is defined by an edge of an implanted region.
 6. The electronic device as recited in claim 1, wherein the active area boundary is straight and oriented orthogonally to the drain finger.
 7. The electronic device as recited in claim 1, further comprising a plurality of source contacts connected to the source electrode and a plurality of drain contacts connected to the drain finger, wherein the drain contact nearest the active area boundary extends further toward the active area boundary than does the source contact nearest the active area boundary.
 8. The electronic device as recited in claim 1, further comprising a plurality of source contacts connected to the source electrode and a plurality of drain contacts connected to the drain finger, wherein the isolation region blocks conduction between the source electrode and the drain contact nearest the drain fingertip sidewall through at least a 150 degree arc around the drain contact.
 9. The electronic device as recited in claim 1, wherein the isolation region includes two segments that run parallel to the drain finger, and a third segment connected to the first and second segments.
 10. The electronic device as recited in claim 1, wherein the drain fingertip sidewall is rounded between the first and second parallel sides.
 11. The electronic device as recited in claim 1, wherein the group IIIA-N barrier layer comprises AlGaN and the group IIIA-N active layer comprises GaN.
 12. The electronic device as recited in claim 1, wherein the group IIIA-N barrier layer comprises InAlGaN.
 13. A method of forming an electronic device, comprising: forming an active area comprising a group IIIA-N barrier layer over a group IIIA-N active layer; forming a source electrode over the active area, the source electrode having a first portion and a second portion, the first and second portions being parallel and spaced apart, and a third portion connecting the first and second portions; forming a drain electrode formed over the active area, the drain electrode having a drain finger located between the first and second source electrode portions, the drain finger having first and second parallel sides connected via a drain fingertip sidewall; and forming an isolation region, an active area boundary between the active area and the isolation region being located between the drain fingertip sidewall and the third portion of the source electrode.
 14. The method as recited in claim 13, further comprising forming a mesa structure comprising the barrier layer, wherein the active area boundary is defined by an edge of the mesa structure.
 15. The method as recited in claim 14, further comprising rounding the edge of the mesa structure by grayscale lithography.
 16. The method as recited in claim 13, wherein the active area boundary is located between the drain fingertip sidewall and a drain contact connected to the drain finger.
 17. The method as recited in claim 13, wherein the active area boundary is defined by an edge of an implanted region.
 18. The method as recited in claim 13, wherein the active area boundary is straight and oriented orthogonally to the drain finger.
 19. The method as recited in claim 13, further comprising forming a plurality of source contacts connected to the source electrode and a plurality of drain contacts connected to the drain electrode, wherein a drain contact nearest the active area boundary extends further toward the active area boundary than does the source contact nearest the active area boundary.
 20. The method as recited in claim 13, wherein the isolation region includes two segments that run parallel to the drain finger, and a third segment connected to the first and second segments. 